This application proposes to investigate hypotheses on the associations of long-term dietary intake of a number of micro nutrients and dietary fats with incident Alzheimer's disease and cognitive decline in an existing biracial population study, the Chicago Health and Aging Project (CHAP). Recent findings by this group of investigators and others suggest that dietary intake of folate, vitamin B-12, and vitamin B-6 may be protective against incident Alzheimer's disease. Others propose that the mechanism for protection by these micronutrients may be through reduction in the accumulation of homocysteine. In addition, we found that intake of antioxidant nutrients and dietary fat composition may be important in the development of Alzheimer's disease, although associations for intakes of polyunsaturated and monounsaturated fats, the n-3 fatty acids, alpha-linolenic acid and eicosapentaenoic acid, vitamin C, and vitamin E supplement use were unclear or marginally significant. The CHAP study provides many essential elements to test these dietary hypotheses including, a prospective design, a validated dietary assessment tool, structured clinical evaluation for unbiased case detection, and up to 15 years of cognitive change data on more than 6,000 persons. However, because the ongoing CHAP study is being greatly expanded to include newly age-eligible cohorts as the original cohort decreases in size due to mortality, and the only existing dietary data is at baseline on the original cohort, it is not currently possible to take advantage of the increased statistical power and longer follow-up of the expanded study. Therefore, in order to account for intraindividual variation in dietary intake over the long follow-up period, and to test conclusively these new and exciting dietary hypotheses, the proposed study will: 1) conduct repeated dietary assessments in the entire CHAP study community to accurately measure long-term intake, and 2) conduct biochemical analyses of banked blood samples for plasma homocysteine concentration. The proposed application should help to answer important scientific questions on the dietary prevention of a debilitating disease that affects a large number of older persons.